This invention relates to a method for enhancing the signal-to-noise ratio in a suite of seismic traces and more particularly to the enhancement of the primary-to-multiple reflection ratio in the seismic traces.
In seismic exploration, seismic energy is generated at a shotpoint at or near the surface of the earth, is reflected from subsurface interfaces between layers of the earth, and is received by a spread of geophones on the surface of the earth. The geophone signals are recorded in the form of a suite of seismic traces. One common method of seismic exploration is known as common depth point exploration. In this method a number of seismic traces are recorded representing seismic energy reflections from a common reflecting point. Referring to FIG. 1, seismic energy, produced at each of a plurality of spaced shotpoints SP.sub.1 -SP.sub.n, is reflected from a subsurface interface at a common depth point (CDP) and is received at a plurality of spaced locations X.sub.1 -X.sub.n for the production of a suite of seismograms. Such a suite of four seismic traces S.sub.1 -S.sub.4 is illustrated in FIG. 2. The ordinate in FIG. 2 represents the horizontal spacing X.sub.1 -X.sub.4 of the receivers which produce the seismograms S.sub.1 -S.sub.4. The primary reflection signals P.sub.1 -P.sub.4 and the multiple reflection signals M.sub.1 -M.sub.4 received from the common depth point reflecting surface are indicated as falling along the curves H and K respectively. Curves H and K represent the arrival time from trace to trace of each of the reflection signals in the suite of seismograms.
One technique for improving the signal-to-noise ratio in such a suite of seismograms makes use of the normal moveout correction for enhancement of primary-to-multiple reflection ratios. Reference is made to U.S. Pat. No. 4,209,854 to W. H. Ruehle for a detailed description of such technique. Briefly, however, acoustic velocity is determined from seismograms by use of the relationship: EQU T.sup.2 =T.sub.o.sup.2 +4h.sup.2 /V.sub.sk.sup.2 ( 1)
where,
T is the time of the reflection on a particular seismic trace, PA1 T.sub.o is the time of that same reflection on an idealized seismic trace with zero source-to-receiver offset, PA1 h is one-half the horizontal distance between the shotpoint and the receiver producing the particular seismic trace, and PA1 V.sub.sk is the acoustic stacking velocity characteristic of the earth medium and the dip of the structure.
The time shift from trace to trace of the relfections from the common depth point surface is commonly known as normal moveout and is specified by the function: EQU .DELTA.T=T.sub.o -.sqroot.T.sub.o.sup.2 +4h.sup.2 /V.sub.sk.sup.2
The traces are time shifted one with respect to another based on different assumed values of average medium velocity. For each reflection, a correlation operation indicates the semblance or signal power between the traces for the different values of assumed average velocity. A maximum in the signal power indicates average velocity to that reflection as illustrated in FIG. 3. This is carried out for each reflection on the suite of seismograms so that average medium velocity to each of these reflectors is determined. The seismic traces are then stacked for the determined average medium velocities to produce a seismic record having enhanced signal-to-noise ratio.